At what rate could AI theoretically self-improve?
The processing power might still follow the Moore's law, but it is basically an economical law, although it has worked surprisingly well considering the physical limitations of technology. An Artificially Intelligent agent could focus on manipulating the market, research possible hardware improvements, allocate human resources more effectively to the problem, etc.

I would expect some improvement in the AI algorithm design itself, which will not influence the number of operations available, but the number of operations required to complete a task. By using a better heuristics, the AI algorithm will consume less brute force to complete the task, thus being able to complete more tasks given the same number of operations. Sorry if I misunderstood the question. Thanks for the improvement.